1. Field of the Invention
The present invention relates to an optical module that includes a light-emitting device and a lens element on which signal light impinges from the light-emitting device, and a method of manufacturing the same.
2. Description of the Background Art
A transmissive type of diffraction lens element is known which has its successive-plane structure causing a diffraction phenomenon to thereby control incident light. Such a diffraction lens element is mounted, for example, on a substrate and is used in an optical module as a component that collects light emitted from a semiconductor laser, or laser diode, i.e. an optical signal.
In one known configuration, the substrate of an optical module has a groove cut therein with a cross section of, for example, V shape, and the diffraction lens element is provided on the substrate so that its light-incident plane is orthogonal to the direction in which the groove extends, see, for example, U.S. patent application publication No. US2006/0115207 A1 to Uekawa et al.
It is also known in the art that in a method of manufacturing an optical module suitably applied to an optical coupler system which includes a semiconductor laser emits an optical signal, which is then collected by a first lens, through which the optical signal passing is then collected by the second lens, through which the optical signal passing is then incident on a single-mode optical fiber, high mounting accuracy is required particularly in order to mount the first lens on the surface of the substrate as a diffraction lens element, see, for example, Hironori Sasaki et al., “Packaging Technologies for Precise Alignment of Light Sources and Silicon Microlenses,” Journal of Japan Institute of Electronics Packaging, Vol. 5, No. 5, pp. 466-472 (2002).
High mounting accuracy is required in order that the semiconductor laser has its laser beam spot size diameter very much different from the spot size diameter of the single mode fiber. Generally, the assembling of diffraction lens elements is carried out to adjust the alignment thereof without operating the semiconductor laser, i.e. the passive alignment assembling. The mounting accuracy is thus dependent upon the dimensional accuracy of alignment marks provided with accuracy in advance, the diffraction lens element or the like.
In the above exemplified configuration of the optical module, i.e. where the diffraction lens element is provided on the substrate so that its light-incident plane is orthogonal to the extending direction of the groove, however, a problem arises that the transmissive diffraction lens element, particularly when made of silicon, has its surface on the light-incident side formed with a convex pattern which is almost the same in height as the reflective diffraction lens element and has its surface on the light-emanating side, opposite to the incident side formed flat so that part of the incident light, i.e. signal light to the diffraction lens is inevitably reflected and returned to the light-emitting device serving as an optical source.
The part of light reflected and returned to the light-emitting device, i.e. a so-called fed-back light may then provide unstable oscillation of the light-emitting device. The fed-back light may also cause a so-called feedback light noise, which may deteriorate the properties of the signal light.